In this episode of Rationally Speaking, Caltech physicist Sean Carroll describes an "embarrassing" state of affairs in modern physics: that we still don't know how to interpret quantum mechanics, almost a century after its discovery. Sean explains why he thinks the "Many Worlds Interpretation" (MWI) is the most plausible one we've got, and Julia explores his thoughts on questions like: Can MWI be tested? Is it "simpler" than other interpretations, and why? And does MWI threaten to destroy our systems of ethics?

Sean Michael Carroll is a research professor in the Department of Physics at the California Institute of Technology. He is a theoretical cosmologist specializing in dark energy and general relativity.

This live episode of Rationally Speaking, taped at the 2015 Northeast Conference on Science and Skepticism, is a special one: it's Massimo's last episode as co-host! He and Julia look back over their history together and discuss which topics they've changed their mind about since the podcast began.

The Amazing Randi, famous magician and a pioneer of Skepticism, joins this episode of Rationally Speaking for a conversation about the past and future of the Skeptic movement. Massimo and Julia's questions for Randi include: Do you think Skepticism has shaped public opinion in any significant ways? What do you want the JREF to look like several years from now? And what have you changed your mind about, and why?

Do atheists need their own 10 commandments? What would such a thing look like? In this episode, Julia and Massimo discuss a recent attempt to define some secular commandments. They debate the relevance of particular commandments, like "All truth is proportional to the evidence," and the purpose of the project overall, and address some criticism of the commandments.

Atheists often take it as a given that the world would be better off without religion. But what does the evidence so far really say? In this episode, Massimo and Julia discuss a recent article in the Skeptical Inquirer presenting research that shows a moderate correlation between religiosity and prosocial traits like altruism. Should we doubt the research? And if not, are there other reasons to suspect that religion's net effect on the world is negative?

This episode marks the fifth anniversary of the Rationally Speaking podcast! To commemorate the occasion, Massimo and Julia hold a live-streaming Q&A in which they respond to questions submitted via Twitter. Topics include: What's the best book to read to improve your rationality? What's the biggest problem with the skeptic community? How can we get politicians to be reasonable? And how can you be so sure that other people exist? Halfway through the show, Massimo makes a surprising and poignant announcement.

Feynman famously said that a philosopher of science is as much use to scientists as an ornithologist is to birds. This episode of Rationally Speaking features philosopher of physics Elise Crull, who explains why Feynman is misguided, and what philosophers have to say about important issues in physics -- like quantum mechanics, physical laws, and whether anything "really" exists at all.

Can it be rational to believe conspiracy theories? On this episode of Rationally Speaking, Julia and Massimo welcome Prof. Preston Bost, a professor of psychology at Wabash College who investigates what kinds of people latch onto conspiracy theories, and why. The three discuss evolutionary reasons for conspiracy theories' appeal, and ask: how do you determine whether a belief is "rational," anyway?

People have been keeping track of their moods, sleeping, dietary habits and more for hundreds of years -- Benjamin Franklin famously recorded instances of his virtues and vices. But only in the last decade has the rise of smartphones and fast computing created the new "Quantified Self" movement in which some people are trying to mine their own data for insights about how to be happier and more effective. In this episode, Massimo and Julia discuss self tracking -- what you can learn from it, and what its pitfalls might be.

Did you miss International Stoic Week this year? Well, it's not too late to catch Massimo and Julia's analysis of the ancient philosophy of stoicism, which advocates (among other things) practicing mindfulness, accepting the things you can't change, and regulating negative emotions. Come hear the results of Massimo's experimentation with stoicism and listen to him and Julia debate several potential problems with the philosophy.